Dryers are typically used to remove moisture from fine, dusty particulate matter such as carbon black or cement particles. For example U.S. Pat No. 3,333,344 discusses the use of rotary dryers in the manufacture of carbon black. Rotary dryers usually have a revolving cylindrical shell which is a vessel that is enclosed by a furnace housing. The material to be dried is inside the shell. A heating medium such as hot gases surrounds the shell and is contained in the furnace housing. Heat is transferred from the heating medium to the shell by radiation and convection. Heat is then transferred from the heated shell to the material to be dried that is inside the shell by conduction and radiation. The thermal energy which is transferred to the material to be dried that is inside the shell evaporates the liquid from the material. The only gases that are flowed inside the shell are those used to purge vapors of the evaporated liquid. Lifting vanes, often simply called lifters, are attached to the inner periphery of the shell to lift and shower the material to be dried. The showering increases the amount of the surface area of the material to be dried that is exposed to heat and to purge gases.
The material to be dried in standard rotary dryers is thus only indirectly heated. The heating medium is physically separated from the material to be dried by the wall of the shell of the dryer. The heating medium heats the shell, and then the heated shell heats the material to be dried. This indirect mode of heat transfer results in a low dryer thermal efficiency. The dryer thermal efficiency is the fraction of the total energy supplied that heats and evaporates liquid from the material to be dried. Various techniques have been used to improve the thermal efficiency of rotary dryers. For instance, the lifters have been modified to increase contact between the material to be dried and the wall of the shell of the dryers. See U.S. Pat. No. 3,333,334. Also, one end of the shells of rotary dryers has been enlarged to increase material hold-up and to increase the amount of time that a given particle of material to be dried is in contact with the hot shells of the dryers. Also, indirectly heated tubes have been installed in dryer shells. These tubes extend through the dryer shell and provide for additional indirect heat transfer from the heating medium into the material to be dried. The heating medium heats the tubes that pass through the dryer shell, and the heated tubes then pass the thermal energy on to the material to be dried. Although the above and other improvements have been made in the art to improve the thermal efficiency of rotary dryers, there is still significant room for improvement.